Stainless steel for hot forging and hot forging method using said steel

ABSTRACT

A micro-alloyed austenitic stainless steel for hot forging is provided. The micro-alloyed austenitic stainless steel has the following composition in weight %: 16.0%≦Cr≦25.0%; 8.0%≦Ni≦25.0%; traces≦Mo≦6.0%; traces≦C≦0.08%; traces≦Si≦1.0%; traces≦Mn≦3.0%; traces≦P 21 0.045%; traces≦S≦0.035%; traces≦Cu≦2.0%; traces≦N≦0.2%; traces≦V≦0.3%; traces≦Ti≦0.3%; traces≦B≦0.005%; traces≦W≦3.0%; 0.015%≦Nb≦0.100%, the remainder being iron and inevitable manufacturing impurities.

The present invention relates to stainless steel for the hot forging of large-size parts.

There is a need for the production of metal parts of large size that are not only corrosion-resistant with strong mechanical properties in particular at high temperature, but which also have good ultrasonic inspectability.

Ultrasound permeability is much dependent on grain size. If this microstructural characteristic becomes too large, permeability vanishes and some defects which may be contained in the part are no longer detectable. In addition the tensile mechanical properties are degraded if grain size becomes too large.

BACKGROUND

For example nuclear reactor boilers contain parts of large dimensions and complex geometry. These parts may be tubular branches for example of the primary cooling circuit having a very wide diameter and provided with branch points.

It is possible to obtain large-size parts by hot forging a steel ingot weighing several tens of tonnes and then machining the forging.

SUMMARY OF THE INVENTION

Nevertheless, the hot forging of such parts requires the ingot to be held at a high temperature over a long period which may last several days. This may affect the microstructure of the steel via excessive enlarging of the grains. The mechanical properties of the part and its suitability for ultrasound inspection are thereby penalised.

It is difficult to keep these aspects under control throughout hot forging. Any non-conformity of the part with specifications that is detected at the end of manufacture after one or more days of hot forging, and after an extremely long machining step, may lead to the part being rejected which represents a major loss.

It is one of the objectives of the invention to allow the manufacture of parts in stainless steel by hot forging, these parts having good ultrasonic inspectability.

To meet this objective a micro-alloyed austenitic stainless steel for hot forging is provided, having the following composition in weight %:

16.0%≦Cr≦25.0%;

8.0%≦Ni≦25.0%;

traces≦Mo≦6.0%;

traces≦C≦0.08%;

traces≦Si≦1.0 %;

traces≦Mn≦3.0%;

traces≦P≦0.045%;

traces≦S≦0.035%;

traces≦Cu≦2.0%;

traces≦N≦0.2%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%,

the remainder being iron and inevitable manufacturing impurities.

According to other embodiments, the steel comprises one or more of the following characteristics taken alone or in any technically possible combination:

the niobium content (Nb) is 0.030 weight % or higher, in particular 0.035 weight % or higher;

the niobium content (Nb) is lower than 0.050 weight %, in particular 0.045 weight % or lower;

the carbon content (C) is lower than 0.05 weight %, preferably it is 0.02 weight % or lower;

the chromium content (Cr) is lower than 23 weight %;

the phosphorus content (P) is 0.04 weight % or lower and/or the sulfur content (S) is 0.03 weight % or lower;

the nitrogen content (N) is 0.1 weight % or lower;

the steel has the following composition in weight %:

16.0 %≦Cr≦25.0 %;

8.0%≦Ni≦25.0%;

traces≦Mo≦6.0%;

traces≦C≦0.08%;

traces≦Si≦1.0 %;

traces≦Mn≦3.0%;

traces≦P≦0.04%;

traces≦S≦0.03%;

traces≦Cu≦2.0%;

traces≦N≦0.1%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100 %,

the remainder being iron and inevitable manufacturing impurities;

the steel has the following composition in weight %:

17.0%≦Cr≦19.0%;

8.0%≦Ni≦10.0%;

traces≦Mo≦6.0%;

traces≦C≦0.08%;

traces≦Si≦1.0 %;

traces≦Mn≦2.0%;

traces≦P≦0.04%;

traces≦S≦0.03%;

traces≦Cu≦2.0%;

traces≦N≦0.1%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015 %≦Nb≦0.100%;

the remainder being iron and inevitable manufacturing impurities;

the steel has the following composition in weight %:

18.0%≦Cr≦20.0 %;

8.0%≦Ni≦12.0%;

traces≦Mo≦6.0%;

traces≦C≦0.03%;

traces≦Si≦1.0 %;

traces≦Mn≦2.0%;

traces≦P≦0.04%;

traces≦S≦0.03%;

traces≦Cu≦2.0%;

traces≦N≦0.1%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%,

remainder being iron and inevitable manufacturing impurities;

the steel has the following composition in weight %:

16.0%≦Cr≦18.0%;

10.0%≦Ni≦12.5%;

2.0%≦Mo≦2.5%;

traces≦C≦0.08%;

traces≦Si≦1.0 %;

traces≦Mn≦2.0%;

traces≦P≦0.04

traces≦S≦0.03%;

traces≦Cu≦2.0%;

traces≦N≦0.2%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%,

the remainder being iron and inevitable manufacturing impurities;

the steel has the following composition in weight %:

16.0%≦Cr≦18.0%;

10.0%≦Ni≦14.0%;

2.0%≦Mo≦3.0%;

traces≦C≦0.02%;

traces≦Si≦1.0 %;

traces≦Mn≦2.0%;

traces≦P≦0.04%;

traces≦S≦0.03%;

traces≦Cu≦2.0%;

traces≦N≦0.1%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%,

the remainder being iron and manufacturing impurities;

the steel has the following composition in weight %:

19.0%≦Cr≦23.0 %;

23.0%≦Ni≦25.0%;

4.0%≦Mo≦5.0%;

traces≦C≦0.02%;

traces≦Si≦1.0 %;

traces≦Mn≦2.0%;

traces≦P≦0.045%;

traces≦S≦0.035%;

1.0%≦Cu≦2.0%;

traces≦N≦0.1%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%,

the remainder being iron and inevitable manufacturing impurities.

The invention also concerns a method for the hot forging of a part from an ingot of stainless steel such as defined above.

According to other embodiments, the method comprises one or more of the following characteristics taken alone or in any technically possible combination:

the ingot has an initial weight of 50 tonnes or higher, in particular 100 tonnes or higher;

the ingot is hot forged at a temperature of between 1300° C. and 1050° C., in particular at a temperature of between 1250° C. and 1150° C.;

hot forging is carried out for a period of time longer than 24 hours, in particular a period of time longer than 36 hours.

BRIEF SUMMARY OF THE DRAWINGS

The invention and its advantages will be better understood on the reading the following description given solely as an example and with reference to the appended drawings in which:

FIG. 1 is a schematic view illustrating steps of a method for manufacturing a part of large size by hot forging an ingot of stainless steel, more particularly a tubular element of the primary piping of a nuclear plant;

FIG. 2 is a functional graph illustrating the temperature of the stainless steel during the manufacture of the part.

DETAILED DESCRIPTION

The manufacturing method comprises a preliminary step to prepare the stainless steel which applies conventional processes and equipment used in an electric steel plant (electric furnace, refining devices, optionally re-melt devices) and the casting of the liquid metal into an ingot mould to solidify an ingot 2 that is to be forged.

The manufacturing method comprises a hot working step whereby the ingot 2 obtained is hot forged.

To obtain the desired part of large dimensions the ingot 2 gas a weight of several tens of tonnes, in particular more than 50 tonnes and more particularly more than 100 tonnes. To produce a primary inlet or outlet branch of a nuclear reactor boiler provided with branch connections, an ingot of over 150 tonnes is routinely used, in particular an ingot of about 170 tonnes.

In the example illustrated in FIG. 1, the ingot 2 is of truncated cone shape with a diameter D in the order of 2566 mm and height in the order of 3735 mm.

The hot working process comprises heating of the ingot 2 to an initial temperature of between 1300 ° C. and 1050° C. and the hot forging of the ingot in successive steps to obtain a forging which forms the blank 4 of the part to be obtained.

FIG. 1 illustrates different steps of hot forging. The blank 4 finally obtained is in the shape of an elongate element of variable diameter. For example the diameter varies between 1050 mm and 2000 mm.

The manufacturing process finally comprises a machining step whereby the blank 4 is machined, for example to form ducts in the blank 4, in particular a main duct and branch ducts leading into the main duct. Having regard to the dimensions of the part, machining typically takes several days.

FIG. 2 illustrates the temperature of the blank during hot forging as a function of time. A first curve C1 illustrates the trend in temperature at the core of the blank, curve C2 illustrates the trend in temperature at one quarter depth of the blank, curve C3 illustrates the temperature at half the depth of the blank and curve C4 illustrates the temperature on the surface.

The hot forging operations on an ingot of heavy weight, in the order of several tens of tonnes, last a long period of time. Typically these operations last more than 24 hours, in particular more than 48 hours and in some cases about ten days.

Since the blank gradually cools throughout forging it is re-heated if its temperature falls too low.

This hot forging can therefore be carried out in numerous steps alternately comprising hot working steps E1 during which the blank 4 cools and re-heating steps E2 during which the blank is heated. FIG. 2 illustrates two forging steps each followed by a re-heating step.

The temperature is between a reheating temperature in the order of 1050° C. to 1300° C. and an end-of-forging temperature which varies in the thickness of the part and which may be in the order of 700° C. on the surface (curve C4).

During these hot forging operations, the stainless steel in held at a high temperature and over a long period of time, which may lead to irreversible changes in the microstructure of the stainless steel that are difficult to control, with resulting uncertainty that the part obtained will conform to specifications and is able to undergo ultrasonic testing.

In particular, the maintaining at high temperature for a long period promotes grain growth. Grains that are too large in size have an influence on the mechanical properties of the steel at the end of manufacture and on the mechanical properties of the part. In addition, grains that are too large are incompatible with ultrasonic testing as required in particular for the parts of nuclear reactor boilers.

The stainless steel used in the method of one embodiment of the invention is hot forged, micro-alloyed, austenitic stainless steel having the following composition in weight %:

16.0%≦Cr≦25.0%;

8.0%≦Ni≦25.0%;

traces≦Mo≦6.0%;

traces≦C≦0.08%;

traces≦Si≦1.0 %;

traces≦Mn≦3.0%;

traces≦P≦0.04%;

traces≦S≦0.03%;

traces≦Cu≦2.0%;

traces≦N≦0.1%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%,

the remainder being iron and inevitable manufacturing impurities.

This austenitic stainless steel has good mechanical properties, in particular satisfactory yield strength whilst avoiding grain growth even if held at a high temperature over a long period for hot forging.

Chromium (Cr) in a content of 16 weight % or higher imparts the stainless nature to the steel. It generates a protective passivation film. A chromium content of 25 weight % or lower allows limiting of the onset of intermetallic phases which would weaken the stainless steel.

Molybdenum (Mo) also imparts the stainless characteristic to steel. Molybdenum contributes towards the formation of a passivation film and strengthens this film. In particular it increases resistance to pitting corrosion. The content of 6 weight % of lower prevents the onset of intermetallic phases which could weaken the stainless steel.

Copper (Cu) strengthens corrosion resistance. It has a stabilising effect on the passivation film.

Nickel (Ni) promotes the onset of austenitic structures. A content of 8 weight % or higher allows austenitic steel to be obtained having good mechanical properties, in particular a very good compromise between yield strength and elongation. A content of 25 weight % or lower allows a balance to be obtained between the chromium and nickel whilst limiting the amount of nickel which is a costly element.

Manganese (Mn) allows trapping of sulfur in the form of sulfur precipitates. It also promotes the onset of austenitic structures and allows limiting of the nickel content.

Tungsten (W) has the same function as molybdenum. Tungsten is optional. The use of tungsten in addition to molybdenum allows limiting of the amount of molybdenum. Tungsten has a significant effect on corrosion resistance on and after a content of 0.5 weight %.

The presence of carbon (C) is inevitable. A content of 0.08 weight % or less preferably 0.05% or less, allows limiting of the formation of the chromium carbides which deplete the metal matrix of chromium and reduce corrosion resistance. It also limits the formation of niobium carbonitrides (Nb), in particular at the end of solidification (primary carbonitrides) which risk degrading some mechanical properties.

The presence of nitrogen (N) is inevitable. The limiting of nitrogen to a content of 0.1 weight % of less allows limiting of the excessive formation of carbonitrides, in particular the excessive formation of niobium carbonitride.

Silicon (Si), phosphorus (P) and sulfur (S) are inevitable and result from the process of steel making

Niobium (Nb) allows limitation of grain growth. It has been observed that niobium reduces the hot recrystallization rate of steel both during forging (dynamic recrystallization) and during the reheating phases (static recrystallization). In addition niobium reduces grain growth rate during the long period of maintained high temperature when forging the steel. Having regard to the manufacturing time of a large-size forged part (typically several days) this moderating effect on grain size is most beneficial.

The niobium content of 0.015 weight % or higher allows satisfactory limitation of grain growth during hot forging, in particular when hot forging of a part from a steel ingot weighing several tens of tonnes. Advantageously the niobium content is 0.030 weight % or higher.

If the niobium content is too high there is a risk of the formation of large-size precipitates of niobium carbonitride, in particular towards the end of ingot solidification. Such precipitates risk degrading the mechanical properties of the steel.

A niobium content limited to 0.100 weight % allows satisfactory grain refining to be obtained by limiting the formation of niobium carbonitride precipitates. Preferably the niobium content is 0.050% or lower.

In one particular embodiment the niobium content is between 0.030% and 0.050% which allows satisfactory grain refining whilst preserving the mechanical properties of the steel in particular its yield strength. In one preferred embodiment it is between 0.035% and 0.045%. In one particular embodiment it is about 0.040%.

Vanadium (V) and titanium (Ti) are carbide-forming elements which may cause the precipitation of vanadium carbides or titanium carbides which trap the carbon and limit the formation of chromium carbide. The formation of such precipitates increases the mechanical properties of the stainless steel, in particular yield strength (Rm). Vanadium has a significant effect on and after a content of 0.05 weight %. Titanium has a significant effect on and after a content of 0.02 weight %.

Boron (B) allows an improvement in the mechanical properties of the steel, in particular its yield strength. Boron has a significant effect on and after a content of 0.0015 weight %.

In one embodiment the stainless steel has the following composition in weight %:

17.0%≦Cr≦19.0%;

8.0%≦Ni≦10.0%;

traces≦Mo≦6.0%;

traces≦C≦0.08%;

traces≦Si≦1.0 %;

traces≦Mn≦2.0%;

traces≦P≦0.04%;

traces≦S≦0.03%;

traces≦Cu≦2.0%;

traces≦N≦0.1%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%;

the remainder being iron and inevitable manufacturing impurities.

This stainless steel corresponds to type 304 steel as per the AISI standard (American Iron and Steel Institute).

In one embodiment, the stainless steel has the following composition in weight %:

18%≦Cr≦20 %;

8%≦Ni≦12%;

traces≦Mo≦6.0%;

traces≦C≦0.03%;

traces≦Si≦1.0 %;

traces≦Mn≦2.0%;

traces≦P≦0.04%;

traces≦S≦0.03%;

traces≦Cu≦2.0%;

traces≦N≦0.1%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%,

the remainder being iron and inevitable manufacturing impurities.

This stainless steel corresponds to type 304L steel as per the AISI standard (American Iron and Steel Institute). Its composition differs from the composition of grade 304 steel in particular through its lower carbon content. Grade 304L steel has higher corrosion resistance than 304 steel.

In one embodiment, the stainless steel has the following composition in weight %:

16.0%≦Cr≦18.0%;

10.0%≦Ni≦12.5%;

2.0 %≦Mo≦3%;

traces≦C≦0.08%;

traces≦Si≦1.0 %;

traces≦Mn≦2.0%;

traces≦P≦0.04%;

traces≦S≦0.03%;

traces≦Cu≦2.0%;

traces≦N≦0.2%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%,

the remainder being iron and inevitable manufacturing impurities.

This stainless steel corresponds to steel of 316 type in accordance with the AISI standard.

In one embodiment, the stainless steel has the following composition in weight %:

16.0%≦Cr≦18.0%;

10.0%≦Ni≦14.0%;

2.0%≦Mo≦3.0%;

traces≦C≦0.02%;

traces≦Si≦1.0 %;

traces 23 Mn≦2.0%;

traces≦P≦0.04%;

traces≦S≦0.03%;

traces≦Cu≦2.0%;

traces≦N≦0.1%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%,

the remainder being iron and inevitable manufacturing impurities.

This stainless steel corresponds to type 316L steel according to the AISI standard.

In one embodiment, the stainless steel has the following composition in weight %:

19.0%≦Cr≦23.0%;

23.0%≦Ni≦25.0%;

4.0%≦Mo≦5.0%;

traces≦C≦0.02%;

traces≦Si≦1.0 %;

traces 23 Mn≦2.0%;

traces≦P≦0.045%;

traces≦S≦0.035%;

1.0%≦Cu≦2.0 %;

traces≦N≦0.1%;

traces≦V≦0.3%;

traces≦Ti≦0.3%;

traces≦B≦0.005%;

traces≦W≦3.0 %;

0.015%≦Nb≦0.100%,

the remainder being iron and inevitable manufacturing impurities.

This stainless steel corresponds to A904L as per the AISI standard. This steel has particularly high corrosion resistance, in particular higher than that of 304, 304L, 316, 316L steel grades.

In each of the different aforementioned embodiments, corresponding to stainless steel grades 304, 304L, 316, 316L and 904L, the niobium content is preferably 0.030% or higher, preferably it is 0.0035% and/or 0.050% or lower, preferably 0.045%. In one particular embodiment it is about 0.040%.

The table below gives the analyses of niobium-containing micro-alloyed stainless steels which allowed evidencing of the beneficial effect of this element on grain size. The table indicates the weight % composition of each species, the remainder being iron and inevitable manufacturing impurities. The species which are not mentioned in the table (B, W, P . . . ) are only contained in trace form.

Cr Ni Mo Mn N C Nb Ti V Cu Example 1 19.3 9.6 0.35 1.67 0.0671 0.026 0.053 — — — Example 2 19.2 9.6 0.34 1.65 0.0681 0.026 0.016 — — Example 3 17.5 12.2 2.21 1.52 0.0603 0.021 0.043 — — — Example 4 21.8 24.9 4.74 1.81 0.0615 0.027 0.039 — — 1.61

Examples 1 and 2 correspond to niobium-containing micro-alloyed steels of grade 304L according to embodiments of the invention, Example 3 corresponds to niobium-containing micro-alloyed steel of grade 316L according to an embodiment of the invention and Example 4 corresponds to niobium-containing micro-alloyed steel of grade 904L according to an embodiment of the invention.

The following table illustrates conventional grades for comparison.

Cr Ni Mo Mn N C Nb Ti V Cu Reference 1 19.3 9.5 0.31 1.65 0.0675 0.025 0.003 — — — Reference 2 17.7 12.2 2.22 1.61 0.0641 0.022 0.002 — — Reference 3 21.7 25.0 4.71 1.75 0.0621 0.025 0.003 — — —

Examples 1 to 4 differ from conventional grades through their niobium content. It is to be pointed out that the niobium concentration of the five alloys is much lower than the concentration that is required to obtain stabilisation of the steel by precipitation of carbon and nitrogen. The niobium micro-alloy embodiments of the invention target refining of grain size after forging and not stabilisation

Reference 1 is a steel of same composition as in Example 1 but substantially devoid of niobium. Reference 2 is a steel of same composition as Example 3 but substantially devoid of niobium. Reference 3 is a steel of same composition as Example 4 but devoid of niobium.

Hot torsion tests performed in the laboratory allowed evidencing of the positive effect of the niobium micro-alloy:

-   -   slowing of dynamic and static recrystallization kinetics;     -   slowing of rate of grain growth.

Treatment 1 Treatment 2 Treatment 3 Treatment 4 Reference 1 25 μm 137 μm 160 μm 168 μm Example 1 21 μm  85 μm 111 μm 121 μm Example 2 23 μm 115 μm 131 μm 149 μm Reference 2 27 μm 141 μm 170 μm 188 μm Example 3 20 μm  81 μm 110 μm 122 μm Reference 3 25 μm 107 μm 155 μm 180 μm Example 4 21 μm  89 μm 109 μm 125 μm

The table above compares the mean grain size of steels according to Examples 1 to 4 and for the reference steels 1 to 3 under different heat treatments 1 to 4.

This mean size is advantageously measured via image analysis i.e. automatic process which interprets metallographic images in which the grain boundaries are evidenced. Another possible technique is naked eye comparison of microstructure photos with standard images, to obtain a grain size number which is also representative of a mean value (e.g. number 10 corresponds to 10 μm and number 8 to 20 μm). The measured number is then converted to a mean grain size expressed in micrometres.

Treatment 1 corresponds to dynamic recrystallization. Treatment 2 is annealing at 1100° C. for 30 min (static recrystallization). Treatment 3 is annealing at 1100° C. for 5 hours (static recrystallization and grain growth). Treatment 4 is annealing at 1100° C. for 20 hours (static recrystallization and grain growth).

Compared with non-microalloyed steels (references 1, 2 and 3), and when following a comparable hot working schedule, the micro-alloyed steels (Examples 1, 2, 3, 4) exhibit a grain size that is reduced by at least one grain size number. It is therefore a significant reduction that is relevant with regard to ultrasonic permeability when the grain size number is close to zero which is the case for the very large forged parts in austenitic stainless steels. 

1-16. (canceled)
 17. A micro-alloyed austenitic stainless steel for hot forging having the following composition in weight %: 16.0%≦Cr≦25.0%; 8.0%≦Ni≦25.0%; traces≦Mo≦6.0%; traces≦C≦0.08%; traces≦Si≦1.0 %; traces≦Mn≦3.0%; traces 23 P≦0.045%; traces≦S≦0.035%; traces≦Cu≦2.0%; traces≦N≦0.2%; traces≦V≦0.3%; traces≦Ti≦0.3%; traces≦B≦0.005%; traces≦W≦3.0 %; 0.015%≦Nb≦0.100%, the remainder being iron and inevitable manufacturing impurities.
 18. The stainless steel as recited in claim 17 having the following composition in weight %: 17.0%≦Cr≦19.0%; 8.0%≦Ni≦10.0%; traces≦Mo≦6.0%; traces≦C≦0.08%; traces≦Si≦1.0 %; traces≦Mn≦2.0%; traces≦P≦0.045%; traces≦S≦0.035%; traces≦Cu≦2.0%; traces≦N≦0.2%; traces≦V≦0.3%; traces≦Ti≦0.3%; traces≦B≦0.005%; traces≦W≦3.0 %; 0.015%≦Nb≦0.100%; the remainder being iron and inevitable manufacturing impurities.
 19. The stainless steel as recited in claim 17 having the following composition in weight %: 18.0%≦Cr≦20.0 %; 8.0%≦Ni≦12.0%; traces≦Mo≦6.0%; traces≦C≦0.03%; traces≦Si≦1.0 %; traces≦Mn≦2.0%; traces≦P≦0.045%; traces≦S≦0.035%; traces≦Cu≦2.0%; traces≦N≦0.2%; traces≦V≦0.3%; traces≦Ti≦0.3%; traces≦B≦0.005%; traces≦W≦3.0 %; 0.015%≦Nb≦0.100%, the remainder being iron and inevitable manufacturing impurities.
 20. The stainless steel as recited in claim 17 having the following composition in weight %: 16.0%≦Cr≦18.0%; 10.0%≦Ni≦12.5%; 2.0%≦Mo≦2.5%; traces≦C≦0.08%; traces≦Si≦1.0 %; traces≦Mn≦2.0%; traces≦P≦0.045%; traces≦S≦0.035%; traces≦Cu≦2.0%; traces≦N≦0.2%; traces≦V≦0.3%; traces≦Ti≦0.3%; traces≦B≦0.005%; traces≦W≦3.0 %; 0.015%≦Nb≦0.100%, the remainder being iron and inevitable manufacturing impurities.
 21. The stainless steel as recited in claim 17 having the following composition in weight %: 16.0%≦Cr≦18.0%; 10.0%≦Ni≦14.0%; 2.0%≦Mo≦3.0%; traces≦C≦0.02%; traces≦Si≦1.0 %; traces≦Mn≦2.0%; traces≦P≦0.045%; traces≦S≦0.035%; traces≦Cu≦2.0%; traces≦N≦0.2%; traces≦V≦0.3%; traces≦Ti≦0.3%; traces≦B≦0.005%; traces≦W≦3.0 %; 0.015%≦Nb≦0.100%, the remainder being iron and inevitable manufacturing impurities.
 22. The stainless steel as recited in claim 17 having the following composition in weight %: 19.0%≦Cr≦23.0 %; 23.0%≦Ni≦25.0%; 4.0%≦Mo≦5.0%; traces≦C≦0.02%; traces≦Si≦1.0 %; traces≦Mn≦2.0%; traces≦P≦0.045%; traces≦S≦0.035%; 1.0%≦Cu≦2.0 %; traces≦N≦0.2%; traces≦V≦0.3%; traces≦Ti≦0.3%; traces 23 B≦0.005%; traces≦W≦3.0 %; 0.015%≦Nb≦0.100%, the remainder being iron and inevitable manufacturing impurities.
 23. The stainless steel as recited in claim 17 wherein the niobium content (Nb) is 0.030% by weight or higher.
 24. The stainless steel as recited in claim 17 wherein the niobium content (Nb) is 0.035% by weight or higher.
 25. The stainless steel as recited in claim 17 wherein the niobium content (Nb) is lower than 0.050% by weight.
 26. The stainless steel as recited in claim 17 wherein the niobium content (Nb) is 0.045% by weight or lower.
 27. The stainless steel as recited in claim 17 wherein the carbon content (C) is lower than 0.05% by weight.
 28. The stainless steel as recited in claim 17 wherein the carbon content (C) is 0.02% by weight or lower.
 29. The stainless steel as recited in claim 17 wherein the chromium content (Cr) is lower than 23% by weight.
 30. The stainless steel as recited in claim 17 wherein the phosphorus content (P) is 0.04% by weight or lower, and/or the sulfur content (S) is 0.03% by weight or lower.
 31. The stainless steel as recited in claim 17 wherein the nitrogen content (N) is 0.1% by weight or lower.
 32. A hot forging method to forge a part from an ingot of the stainless steel as recited in claim
 17. 33. The hot forging method as recited in claim 32 wherein the ingot has an initial weight of 50 tonnes or higher.
 34. The hot forging method as recited in claim 32 wherein the ingot has an initial weight of 100 tonnes or higher.
 35. The hot forging method as recited in claim 32 wherein the ingot is hot forged at a temperature of between 1300° C. and 1050° C.
 36. The hot forging method as recited in claim 32 wherein the ingot is hot forged at a temperature of between 1250° C. and 1150° C.
 37. The hot forging method as recited in claim 32 wherein the hot forging is conducted for a period of time longer than 24 hours.
 38. The hot forging method as recited in claim 32 wherein the hot forging is conducted for a period of time longer than 36 hours. 